It is often necessary for devices constructed in the same substrate to be electrically isolated from each other. Various structures have been used to provide such an isolation. For example, different dielectric materials have been formed in semiconductor substrates about active and passive elements that need to be isolated and there have been several methods for forming dielectric materials for such applications. Some of these methods involve the oxidation of porous silicon layers to form isolation regions. Method for using such an approach have been disclosed in U.S. Pat. No. 3,640,806 issued on Feb. 8, 1972 to Y. Watanabe et al, U.S. Pat. No. 3,919,060 issued on Nov. 11, 1975 to H. B. Pogge et al, U.S. Pat. No. 4,016,017 issued on Apr. 5, 1977 to J. A. Aboaf et al and U.S. Pat. No. 4,104,090 issued on Aug. 1, 1978 to H. B. Pogge, the later three of which are assigned to the assignee of the present application. The Watenabe et al patent discloses a process which in general consists of masking a silicon substrate, anodizing the substrate to form porous silicon regions in the unmasked areas and exposing the heated substrate to an oxidizing ambient. The porous silicon oxidizes at a rapid rate to form an insulator around monocrystalline silicon regions. The Pogge et al patent describes a method for producing dielectrically isolated regions in a silicon substrate by forming in the substrate high conductivity regions of an opposite conductivity type to that of the substrate to define the areas where dielectric regions are needed. These regions are then anodically etched using a hydrofluoric acid solution to produce regions of porous silicon in the high conductivity regions. These porous silicon regions are then exposed to an oxidizing ambient at an elevated temperature to oxidize the porous silicon regions to form a complete isolation of the monocrystalline silicon regions.
The Aboaf et al patent describes a process for making in a semiconductor structure a pattern of oxidized and densified porous silicon regions for dense isolation. The process involves forming a pattern of porous silicon regions in the surface of a silicon substrate and oxidizing the structure at a temperature sufficient to completely oxidize the porous silicon. The oxidation is selected so that the oxidized porous silicon extends above the surface of the silicon body. The oxidized porous silicon regions are then subjected to a temperature higher than the oxidizing temperature utilized in the previous step to cause the oxidation of the porous silicon.
The Pogge patent discloses a process which includes forming on a P type substrate a P.sup.+ layer and in this P.sup.+ layer an N or P type surface layer and forming openings in the N or P surface layer which reach at least down to the P.sup.+ layer. The structure is then subjected to anodic etching techniques which preferentially attack the P.sup.+ layer to form porous silicon throughout the P.sup.+ layer. The structure is then placed in a thermal oxidation ambient until the porous silicon layer has been fully oxidized to silicon dioxide. The openings through the surface layer are then filled up with oxide to fully isolate the N or P surface layer.
The inventions disclosed and claimed in the aforementioned patents represent truly significant advances in this art as explained in said patents. Our present invention extends the developments in this area of technology and has many advantages and flexibilities. For example, it produces structures having a buried oxidized porous silicon layer which is substantially uniform in thickness and can be made relatively thin and with improved uniformity of silicon porosity under the preselected Si islands.